The present invention relates to an inverter apparatus for controlling the speed of an induction motor variably.
As a method of controlling an inverter for driving the induction motor so that the induction motor is operated at variable speed, there is known a V/f fixed control method of controlling an output voltage (V1) of the inverter in proportion to a primary frequency (f1) of the inverter. This method has a problem that when a load is applied, an induced voltage (Em) of the induction motor is reduced because of a voltage drop across a primary resistance (r1) of the induction motor, so that a magnetic flux of the induction motor is made small and accordingly a maximum torque is reduced.
In order to increase a torque in a low and medium speed area, a general inverter includes torque boost function. When a large start torque is required, a boost voltage is set up to a high voltage in a low speed area and the boost voltage is added to a V/f fixed voltage command (induced voltage command Em*) to produce an output voltage command of the inverter. However, when the boost voltage is increased, over-excitation occurs in no load. When the over-excitation occurs, the magnetic flux of the induction motor is saturated and accordingly an excitation reactance is reduced to thereby increase an excitation current. Consequently, the temperature of the induction motor rises or the current of the inverter is increased excessively, so that there is the possibility that over-current protection function or over-load protection function is operated to be tripped.
A method of suppressing the over-excitation is described in, for example, JP-A-7-163188. In this method, a command for setting up a frequency to zero is issued before start of operation and a DC current is supplied to the induction motor. An output voltage of the inverter at the time that a current of U-phase becomes equal to an equivalent of a design value of the excitation current is set up as a torque boost voltage ΔVz0 at the time that the frequency is 0 Hz.